Protein is an important nutritional component for mammalians. Unlike most plants and microorganisms that can biosynthesize all twenty standard amino acids needed for health and reproduction, typical mammals cannot synthesize all amino acids needed for survival. These amino acids that cannot be synthesized are referred to as essential amino acids and are required to be obtained through the diet. Amino acids are provided in the diet by ingesting protein-containing foods. Digestive enzymes can hydrolyze peptide bonds in the ingested proteins to release individual amino acids. These amino acids are free to be absorbed into the body, providing nutritional benefits.
In addition to providing required nutrients, foods high in protein are increasingly desired by consumers for a variety of others reasons. When consumed, foods high in protein can help satisfy an individual's feeling of hunger for a longer period of time than when consuming comparatively lower protein foods. This helps limit overconsumption of food and can help balance a healthy lifestyle. Further, a high protein diet can support muscle growth and maintenance for active individuals.
Given the nutritional and lifestyle benefits of a high protein diet, food manufacturers are providing increasing numbers of high protein products to the consuming public. Before marketing these products, manufacturers typically first quantify the amount of digestible protein in each product. Different proteins digest to different extents within the mammalian digestion system and, correspondingly, provide different nutritional benefits. It is the amount of digestible protein that is usually reported on food packaging labels.
Traditionally, protein digestibility has been evaluated in vivo using rat subjects to measure the amount of protein digestion that occurs when fed a protein-containing product. The total amount of nitrogen in the food product is measured before being fed to a rat subject and compared to the total amount of nitrogen in the feces produced by the rat. The nitrogen uptake by the rat provides a measure of the amount of digestible protein in the product. While yielding suitable results, this in vivo measurement process is both costly and time consuming. For example, a typical in vivo protein measurement using a rat subject can take 30 days or more to perform.